Automated ultrasonic elasticity image formation with quality measure

ABSTRACT

Image data and E-mode images used in ultrasonic elasticity imaging may be automatically evaluated for quality to provide a single value used as operator feedback or for automatic selection of images for averaging or animation.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States government support awarded bythe following agencies: NIH EB002722, NIH R01 CA100373, andDAMD17-00-1-0596. The United States has certain rights in thisinvention.

CROSS-REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION

The present invention relates to ultrasonic medical imaging devices and,in particular, to a method of automatic elasticity image formation inwhich each elasticity image is accompanied by a quality measure that canbe used for the purpose of operator feedback or automatic imageselection.

Elasticity (E-mode) imaging reveals the stiffness properties of tissue,for example, strain, Poisson's ratio, and Young's modulus. The stiffnessmeasurements may be collected over an area or volume and then mapped toa gray or color scale to form a two- or three-dimensional E-mode image.

In quasi-static elasticity imaging, images of the tissue in differentstates of deformation are obtained by ultrasonic or other imagingdevices. Strain is deduced from two images by computing a gradient indisplacement between the images along any desired direction.Quasi-static elasticity imaging is analogous to a physician's palpationof the tissue during which the physician determines stiffness bypressing the tissue and detecting the amount of tissue yield under thispressure.

The tissue deformation may be obtained manually, for example, by movingthe ultrasonic transducer toward and away from the tissue or through aseparate compressor mechanism or by physiological movement. Manualdeformation of the tissue provides an extremely versatile technique thatcan be used with standard ultrasonic imaging equipment; however, manualdeformation requires considerable operator skill. Ideally, for example,the deformation is at an angle and in an amount to reduce lateral tissueslippage while obtaining appropriate tissue displacement. Too much ortoo little deformation will not yield consistent E-mode image data.

Often E-mode image data is presented as a time series animation so as toprovide an additional dimension of information to the physician. Lowquality images incorporated into this animation can create disruptivebreaks in the animation obscuring the extra dimension of informationhoped to be obtained. Multiple E-mode images may be combined to reduceimage noise or provide E-mode measurements along different axes. Again,low quality images incorporated into this combination decrease thequality of the measurement.

SUMMARY OF THE INVENTION

The present invention provides an automatic method of forming E-modeimages using quality values that can be employed to provide nearreal-time operator feedback or an automatic culling of poor images.Importantly, the method operates quickly and may provide a single“quality value” that can be evaluated automatically against a thresholdand/or unambiguously displayed to an operator to guide the operator'sdeformation technique and/or to train operators. In the preferredembodiment, the single quality value is derived from different metrics,each having different strengths.

Specifically, the present invention may provide an E-mode imagingapparatus having a tissue compressor adapted to apply a varyingdeformation to tissue and an image acquisition system collecting aseries of images of the tissue during different stages of deformation bythe tissue compressor. An electronic computer receives the series ofimages to generate a quality value singly indicating a quality of E-modedata obtainable from a currently acquired subset of the series ofimages. An output of the quality value is provided for use in adjustingor varying deformation to improve the quality of E-mode data.

Thus it is one object of at least one embodiment of the invention todevelop a near real-time E-mode formation method where each E-mode imageassociates with a single scalar measurement that may be used to assessits quality.

It is another object of at least one embodiment of the invention toprovide a real-time single quality measurement that may be used toimprove the acquisition of E-mode image data.

The present invention may form E-mode images using multiple imagesignals (as opposed to derived E-mode images) in such a way thatguarantees all E-mode images reside in the same physical grid. CompositeE-mode images that may have higher signal-to-noise-ratios (SNRs) can beobtained by averaging these E-mode images located in the same physicalgrid without losing spatial resolution. These composite E-mode imagescan be displayed for diagnosis or training.

Thus it is another object of at least one embodiment of the invention toprovide a method of E-mode image formation by which averaging E-modeimages may provide E-mode images with higher signal to noise ratios fordiagnosis or training but are not penalized by the reduction in spatialresolution.

The selection of multiple image signals separated by time (as opposed tothe derived E-mode images) in the present invention may be rapidlydetermined to achieve the highest possible quality in the compositeE-mode image.

Thus it is another object of at least one embodiment of the invention toprovide a method of selecting image signals (as opposed to the derivedE-mode images) under which high quality composite E-mode images can beobtained.

The compressor may be manipulable by an operator and the output may bean operator interface providing a representation of the quality valueselected from the group consisting of: a displayed number, a displayedvisual gauge, a displayed indicator light, and an audio signal.

Thus it is another object of at least one embodiment of the invention toprovide a real-time corrective signal to an operator manually deformingtissue to improve the quality of the data acquired.

The compressor may be an ultrasonic transducer and may also provide echosignals for the image acquisition system.

It is thus another object of at least one embodiment of the invention toprovide a system that may be used with standard ultrasonic acquisitionsystems for elasticity measurements.

The quality value may be derived from a comparison of at least one pairof motion corrected images.

Thus it is another object of at least one embodiment of the invention toprovide a measurement that may be made directly on the image signals (asopposed to the derived E-mode images) to predict the quality ofelasticity information to be obtained therefrom.

The comparison may be a correlation of the motion corrected images.

It is another object of at least one embodiment of the inventiontherefore to provide a simple mathematical technique for evaluatingimages that can be sensitive to the entire image area.

The electronic computer may further process the images to create E-modeimages located in the same physical grid and the quality value may bederived from a comparison of the E-mode images.

Thus it is another object of at least one embodiment of the invention toprovide a measurement that looks directly at the E-mode images to deducetheir quality.

The comparison of the E-mode images may be an evaluation of crosscorrelation or mutual information or other correlation providing amathematical equivalent of these evaluations of the E-mode images.

It is thus another object of at least one embodiment of the invention toprovide a measurement of E-mode images analogous to the measurement ofthe images used to deduce elasticity information.

The comparison of the E-mode images located at the same physical gridusing correlation, mutual information or its mathematical equivalencecan reveal the signal-to-noise-ratio (SNR) in their correspondingcomposite E-mode images.

Thus it is another object of at least one embodiment of the invention tomeasure the signal-to-noise-ratio of the composite E-mode images formedby the current invention.

The quality value, alternatively or in addition, may be derived from anevaluation of information content of at least one E-mode image, forexample, by determining entropy.

It is thus another object of at least one embodiment of the invention toprovide a measurement of E-mode image quality that may be obtained froma single image for rapid determination.

The quality value may be based on a single quality metric or may be acombination of two or more different types of measurements of thequality of the E-mode image information.

Thus it is another object of at least one embodiment of the invention toprovide a single, quality value deriving from the strengths and benefitsof different quality measurement techniques.

The present invention also provides a method of processing a stream ofimage data to produce E-mode measurements including the steps of:evaluating the stream of images to create a set of corresponding qualityvalues indicating a quality of composite E-mode image data obtainablefrom the stream of images, and generating output E-mode images usingonly images associated with quality values over a predeterminedthreshold.

Thus it is another object of at least one embodiment of the invention toprovide for the possibility of automatic selection of images for use ingenerating elasticity data.

The composite E-mode images may generally be used to form an animationor may be further mathematically combined.

Thus it is another object of at least one embodiment of the invention toprovide a method suitable for a variety of different E-mode image outputtechniques.

The evaluation of the stream of images may include evaluation of pairsof images having different time separations and generating for eachpair, and for each time separation, a quality value. The generation ofthe output E-mode images may then select among the time separations togenerate output E-mode images using pairs of images with time separationassociated with quality values over a determined threshold.

It is thus another object of at least one embodiment of the invention toprovide a method of automatically selecting appropriate time separationsto provide improved elasticity data.

The overall evaluation of a stream of E-mode images using quality valuesmay be derived from the quality evaluation for each individual E-modeimage to provide additional feedback to operators.

It is thus another object of at least one embodiment of the invention toprovide an overall measurement for a sequence of E-mode images obtainedfrom image signals (as opposed to the derived E-mode images) acquiredcontinuously.

These particular objects and advantages may apply to only someembodiments falling within the claims, and thus do not define the scopeof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an ultrasonic scanner suitablefor use with the present invention;

FIG. 2 is a flow diagram showing processing of a series of ultrasonicimages obtained from the scanner of FIG. 1 to produce a quality value;

FIG. 3 is a detailed block diagram of a motion correction block of FIG.2 for making motion correction comparison of the image data;

FIG. 4 is a diagrammatic representation of an application of the presentinvention to automatically select pairs of images using deformationvalues to obtain high quality E-mode images;

FIG. 5 a is a diagrammatic representation of the selection of particularpairs of images based on quality values to obtain high quality E-modeimages;

FIG. 5 b is a pictorial representation of the selection of particularE-mode images in a time series by use of a quality value obtained by thepresent invention;

FIG. 6 is a figure similar to that of FIG. 2 showing the preservation ofa single physical grid in the combination of image data; and

FIG. 7 is a diagrammatic representation of the collection of qualitydata for a measure of a combined sequence of E-mode images.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 in a preferred embodiment, an E-mode imagingsystem 10 suitable for use with the present invention provides anultrasonic transducer 12 which may transmit multiple ultrasonic beams 14toward a region of interest 18 within a patient 20. The ultrasonic beams14 produce echoes along different measurement rays passing throughvolume elements within the region of interest 18.

The echoes are received by the transducer 12 and converted to electricalsignals acquired by interface circuitry 22 of a main processor unit 24.The interface circuitry 22 may perform amplification, digitization, andother signal processing on the echo signals as is understood in the art.These digitized echo signals may then be transmitted to a memory 26 forstorage and subsequent processing by a processor 27 as will bedescribed. The processor 27 is preferably an electronic computer, a termwhich, as used herein, encompasses all numeric processing machinesproviding equivalent function including analog and digital computers aswell as specially designed signal processing circuitry.

According to the techniques well known in the art, the stored echofields 28 may each provide a B-mode image and thus will be referred togenerally as image data. Two or more of the echo fields 28 used to formB-mode images are processed to produce an E-mode image 42 according to astored program 33 held in memory. The B-mode images and the E-modeimages 42, both derived from the echo fields 28, may be displayed on anoperator display 32 connected with the processor unit 24 by graphicinterface 34. The operator display 32 may provide a color monitor and/ora sound output according to techniques well known in the art. Thegraphic interface 34 may also accept inputs from the keyboard and/orcursor control device 37.

These features are generally available in commercial ultrasound machinessuch as the Elegra scanner available from the Siemens Medical Solutions,Inc., having a 7.2 MHz linear array transducer operating at 36 MHz insampling frequency.

Referring now to FIGS. 1 and 2, the present invention may be implementedin software of the stored program 33 as a series of functional blocks tonow be described.

Initially, sequential echo fields 28 a through 28 d are analyzed in apairwise fashion (e.g. echo fields 28 a and 28 b, 28 b and 28 c, etc.)to estimate displacements, derive E-mode images and to perform aninitial E-mode image quality assessment. At a first step in the E-modeimage quality assessment, each echo field of the pair, for example, echofields 28 a and 28 b are first received by a data warping block 35 whichwarps the later echo field, in this case, echo field 28 b to produce awarped echo field 28 b′ matching as closely as possible the earlier echofield 28 a. Generally each of the echo fields will have been acquiredwith a different tissue deformation.

Referring also to FIG. 3, the warping block 35 first compares the twoecho fields 28 a and 28 b, as indicated by comparison block 36, todetermine a displacement map 38 indicating relative motion between thetissue of the two echo fields 28 a and 28 b. Calculating such adisplacement map 38 is well understood in the art of E-mode imaging andmay, for example, be done by making local correlations of the echofields 28 a and 28 b within a series of predefined windows over thesurface, or volume, of the data to determine local displacement. As willbe understood to those of skill in the art, the invention may operate ontwo or three dimensional echo field acquisitions.

The displacement map 38 is then further processed by the E-modeestimation block 40 to produce an E-mode image 42 a. E-mode imageformation is well understood in the art of elasticity imaging and may,for example, form strain images by computing the spatial gradient of thedisplacement field or compute Poisson's ratio or elastic moduli withadditional processing, as will be discussed further below.

The displacement map 38 is also received by warper 44 which uses thedisplacements indicated in the displacement map 38 to warp echo field 28b to approximate the echo field 28 a thereby producing motion correctedor warped echo field 28 b′.

Referring again to FIG. 2, the unwarped echo field 28 a and warped echofield 28 b′ may then be compared by correlator 46 and the correlationvalue provides a first quality parameter 48 to be received by combiner50 to be discussed below. Correlation, as is well understood in the artmay measure the sum of the magnitude of the differences between theimages on a point-by-point basis over the entire images. Generally thisfirst quality parameter 48 will be sensitive to errors in thedisplacement map caused by a variety of problems including excessivedeformation that moves corresponding tissue outside the analyses window,or by poorly resolved correlation maxima caused by noise or lack ofstrong echo features in the echo field 28.

This process of determining the first quality parameter 48 is repeatedfor each sequential pair of echo fields 28 to provide a series of firstquality parameters 48 associated with each pair of echo fields 28.

As mentioned above, the E-mode estimation block 40 of the warping block35 also provides the E-mode image 42 a according to techniques wellknown in the art. Each pair of echo fields 28 processed in this mannerwill produce one of a series of E-mode images 42 a, 42 b, etc.

Referring now to FIG. 6, each pair of E-mode images 42, for example,E-mode images 42 a and 42 b are estimated at the same physical grid 43b. One way to accomplish this goal is to use three echo fields, forinstance, echo fields 28 a, 28 b and 28 c. The warping block 35 firstcompares two echo fields 28 a and 28 b at comparison block 36 todetermine a displacement map 38. The echo fields 28 a and 28 b are usedas target and reference echo fields, respectively. That is, thedisplacement map 38 indicates a relative motion from the echo fields 28b and its physical grid 43 b to the echo fields 28 a. Then the E-modeestimation block 40 is used to extract the E-mode image 42 a that ismapped into the physical grid 43 b of the echo field 28 b. As will beunderstood to those of ordinary skill in the art, this use of the samephysical grid is not essential for all embodiments of the invention.

The E-mode image 42 b is obtained from the echo fields 28 b and 28 cfollowing a similar procedure where the echo fields 28 b and 28 c arethe reference and target echo fields respectively to assure that theE-mode image 42 b also located at the physical grid 43 b of the echofield 28 b. The E-mode images 42 a and 42 b can be combined using thecombiner 80 to produce a composite E-mode image 82 b by weightedaveraging. The composite E-mode image 82 b is then on the same physicalgrid 43 b as the reference echo field 28 b. The combination procedure isintended to reduce noise and will not degrade spatial resolution inE-mode images (as opposed to temporal smoothing where E-mode imagesseparated by time and referenced to different physical grids areaveraged directly).

Referring again to FIG. 2, each pair of these E-mode images 42, forexample, E-mode images 42 a and 42 b, are next compared to determinetheir similarity by cross correlation (or mutual information or otherfunctionally equivalent comparison) comparison block 52. Mutualinformation is a well known mathematical technique described, forexample, in Introduction to Statistical Communication Theory, by D.Middleton, New York, John Wiley & Sons, 1991, hereby incorporated byreference. The similarity of the two E-mode images 42 a and 42 b isprovided as second quality parameter 54 for each pair of echo fields 28to be received by combiner 50. The similarity between the two E-modeimages 42 a and 42 b strongly correlates to the E-mode imagesignal-to-noise-ratio (SNR) of the composite E-mode image 82 b (FIG. 6)and the E-mode image SNR can also be used as a second quality parameter54.

Each E-mode image 42 is also evaluated individually for its informationcontent by means of an entropy measurement, for example, Shannon'sentropy also defined in the above-referenced book to provide a thirdquality parameter 56. Generally, maximum entropy is obtained when allgray (or color) values of the image are equally represented whileminimum entropy occurs when the image is of a single brightness. Theentropy of the first E-mode image 42 a of each image pair is determinedby entropy block 51 as third quality parameter 56 received by combiner50. The third quality parameter 56 may be normalized to a quantitybetween zero and one.

Each of these quantitative quality parameters 48, 54 and 56 may becombined by combiner 50 which combines the quality parameters 48, 54 and56 together by an empirically determined equation to produce a singlequality value 60.

One realization may be the products of quality parameters 48, 54 and 56.

This single quality value 60 may be displayed by the processor 27 on theoperator display 32, for example, in the form of a bar display 62 havinga shaded portion 64 that increases to fill the bar of the bar display 62as the quality value 60 increases. In this way, the operator may bepresented with a real-time display of quality value 60 to adjust his orher technique in compressing the tissue using the ultrasonic transducer12.

Alternative forms of representation including numeric displays, forexample, from zero to 100, color displays, tones, or the like may alsobe used.

Referring now to FIG. 5 a, one way of selecting three echo fields, forexample, echo fields 28 a, 28 d and 28 f, may be done by usingmeasurements of quality value 60 obtained as described above. Forexample, a reference echo field 28 d may first be selected. Two seriesof E-mode images can be generated by going backward (for instance, echofields 28 d to 28 c, and echo fields 28 d to 28 b) and forward (forinstance, echo fields 28 d to 28 e, and echo fields 28 d to 28 f) intime. Then the composite E-mode images 82 can be accepted or rejecteddepending on the quality value 60 of that triplet of echo fields 28 andtheir consequent E-mode images 42. Among these echo fields 28 having aquality value 60 greater than the pre-determined threshold 68 for itscorresponding composite E-mode image 82, the triplet that has thehighest quality value 60 will be eventually selected.

Referring now to FIG. 4, in another application of the presentinvention, a series of echo fields 28 a through 28 i may be obtained atdifferent degrees of tissue deformation as indicated by deformationcurve 71 providing field averaged strain of the echo fields 28. Theprocessor 27 executing the stored program 33 receiving echo fields 28may evaluate pairs of echo fields 28 having different time separations.Thus, for example, echo fields 28 a and 28 c may be compared as well asecho fields 28 d and 28 e. Generally, each pair of echo fields willprovide between them a tissue strain value 72 representing thedifference between the values of the deformation curve 71 at the time ofthe acquisition of the particular echo field 28.

The selection of the particular pairs of echo fields 28 a and 28 c, forexample, may be done by finding comparable tissue strain values 72 ormay be done by using a combination of tissue strain and measurements ofquality value 60 obtained as described above. For example, pairs of echofields 28 may first be selected by tissue strain values 72 and thenaccepted or rejected depending on the quality value 60 of that pair ofecho fields 28.

Referring now to FIG. 5 b, the quality value 60 may further be used toselect only those E-mode images 42 having greater than a pre-determinedquality value 60 to be combined by combiner 80 to produce a compositeE-mode image 82. The composite E-mode image may produce a superiorsignal-to-noise ratio when constructed of similar E-mode images 42 ormay provide additional dimensions of E-mode measurement, for example,for deformations along different angles or of different amounts.

In this latter regard, referring to FIG. 2, the quality value 60 may beprovided to a comparator 66 which may compare the quality value 60against a threshold 68 to produce a binary acceptance value 70 that maybe used for eliminating low quality images on an automatic basis.

Referring now to FIG. 5, this same technique may be applied to a seriesof E-mode images 42 a through 42 f as shown in FIG. 5 such as may beoutput as composite E-mode images in the form of an animation. In thiscase, the quality value 60 is compared against the threshold 68 andwhere the quality value 60 drops below the threshold 68 for a particularE-mode image 42 b and 42 e in this example, those E-mode images areeliminated from the animation.

Referring now to FIG. 7, the quality value 60 associated with eachE-mode image 42 may further be used by a sequence combiner 85 (combiningthe quality values 60 of the E-mode images 42) to accumulate the overallquality 86 for a sequence of echo fields 28 acquired continuously. Theoverall quality 86 provides additional feedback to operators or can beused for training purposes.

These techniques are not limited to use with data acquired withultrasound machines, but can be used with other image modalities as willbe understood to those of ordinary skill in the art. It is specificallyintended that the present invention not be limited to the embodimentsand illustrations contained herein, but include modified forms of thoseembodiments including portions of the embodiments and combinations ofelements of different embodiments as come within the scope of thefollowing claims.

1. An E-mode imaging apparatus comprising: a compressor adapted to applya varying deformation to a material to be imaged; an image acquisitionsystem collecting a series of images of the material to be imaged duringdifferent stages of deformation by the compressor; an electroniccomputer receiving the series of images to generate E-mode images with aquality value singly indicating a quality of E-mode data obtainable froma currently acquired subset of the series of images; and an output ofthe quality value in adjusting the varying deformation to improve thequality of E-mode data.
 2. The E-mode imaging apparatus of claim 1wherein the compressor is manipulable by an operator and the output isan operator interface providing a representation of the quality valueselected from the group consisting of: a displayed number, a displayedvisual gauge, a displayed indicator light, an audio signal.
 3. TheE-mode imaging apparatus of claim 1 wherein the quality value is derivedfrom a comparison of at least one pair of motion corrected images. 4.The E-mode imaging apparatus of claim 3 wherein the comparison is acorrelation of the motion corrected images.
 5. The E-mode imagingapparatus of claim 1 wherein the quality value is derived from acomparison of E-mode images.
 6. The E-mode imaging apparatus of claim 5the comparison of E-mode images compares a correlation of the E-modeimages.
 7. The E-mode imaging apparatus of claim 5 the comparison ofE-mode images assesses a signal-to-noise ratio of the E-mode images. 8.The E-mode imaging apparatus of claim 1 wherein the quality value isderived from an evaluation of information content of at least one E-modeimage.
 9. The E-mode imaging apparatus of claim 8 wherein theinformation content is evaluated by determining entropy.
 10. The E-modeimaging apparatus of claim 1 wherein the quality value is a combinationof at least two different types of measurements of the quality of E-modeinformation obtainable from the subset.
 11. The E-mode imaging apparatusof claim 10 wherein the electronic computer further processes the imagesto create strain images and wherein at least one type of measurementevaluates image data and another type of measurement evaluates strainimages.
 12. The E-mode imaging apparatus of claim 11 wherein the qualityvalue is a combination of values obtained from: (1) a correlation valueof motion corrected images; (2) similarity among derived E-mode images;and (3) information content of at least one derived E-mode image. 13.The E-mode imaging apparatus of claim 1 wherein the compressor is anultrasonic transducer manipulable by an operator and the output is anoperator interface providing a representation of the quality valueselected from the group consisting of: a displayed number, a displayedvisual gauge, a displayed indicator light, an audio signal.
 14. A methodof processing a stream of images to produce E-mode measurementscomprising the steps of: (a) evaluating at least one of the stream ofimages and a stream of E-mode images produced from the images to createa set of corresponding quality values indicating a quality of E-modedata obtainable from the stream of images; and (b) generating outputE-mode images using composite strain images associated withpredetermined quality values.
 15. The method of claim 14 wherein step(a) evaluates pairs of images having different time separations andgenerates for each pair and each time separation a quality value, andwherein step (b) selects among time separations to generate outputE-mode images using pairs of images with time separation associated withquality values over a determined threshold.
 16. The method of claim 14wherein output E-mode images form an animation.
 17. The method of claim14 wherein output E-mode images are mathematically combined to produceat least one composite output image.
 18. The method of claim 14 whereinthe images are ultrasonic echo fields.
 19. The method of claim 14wherein the quality values are a combination of at least two differentmethods of measurements of the quality of E-mode information obtainablefrom the stream of images.
 20. The method of claim 19 wherein at leastone method of measurement evaluates motion compensated images andanother method of measurement evaluates E-mode images.
 21. The method ofclaim 14 wherein the quality values are a combination of values obtainedfrom: (1) a correlation assessment of motion corrected images; (2) acorrelation, mutual information or its mathematical equivalent metric ofderived E-mode images; and (3) information content of at least onederived E-mode image.
 22. A method of assigning a quantitative qualityvalue to motion tracking used for E-mode imaging comprising the stepsof: (a) acquiring at least three echo fields of an object in differentstates of deformation; (b) tracking displacement from one echo field asa reference to the other two echo fields as targets at a series of localcomparison regions and obtaining two point-by-point displacement fields;(c) warping at least one of the target echo fields based on one of thepoint-by-point displacement fields; and (d) comparing the reference echofield and the target echo field after warping to produce thequantitative quality value.
 23. A method of providing improved E-modeimages comprising the steps of: (a) acquiring a series of images of anobject under time varying deformation; (b) pairing each image withanother image in the series according to an assessment of image pairingsfor producing E-mode images; and (c) processing the paired images toproduce a series of E-mode images.
 24. A method of providing improvedcomposite E-mode images comprising the steps of: (a) acquiring a seriesof at least a first, second and third time-ordered image of an objectunder time varying deformation, each image acquired relative to aninherent physical image grid; (b) processing the first and second echoimages to produce a first E-mode image indicating elasticity informationrelative to the second physical image grid; (c) processing the secondand third images to produce a second E-mode image indicating elasticityinformation relative to the second physical image grid; and (d)mathematically combining the first and second E-mode images per thesecond physical image grid.
 25. A method of providing a quality valuefor a sequence of images acquired continuously comprising the steps of:(a) determining quality values representing the quality of E-mode imagesobtainable from combinations of the images; and (b) mathematicallycombining these quality values to a single summary quality value.
 26. Amethod of selecting three images from a time series of images to producea composite E-mode measurement comprising the steps of: (a) selecting areference image; (b) creating a first set of E-mode images by pairingimages backward in time; (c) creating a second set of a set of E-modeimages by pairing echo fields forward in time; (d) evaluatingcorresponding quality values indicating a quality of E-mode dataobtainable from possible combinations of elements of these two sets ofE-mode images; and (e) selecting E-mode images with the highest qualityvalues to create a composite E-mode image.
 27. The method of claim 26wherein the composite E-mode image is selected from the group consistingof: a combining of multiple E-mode images to a single image and thecombining of multiple E-mode images to an animated E-mode imagesequence.